The present invention relates to an achromatic lens to be used as a collimator lens, or an objective lens for a telescope. Particularly, the present invention relates to a diffractive-refractive achromatic lens that includes a refractive lens system provided with a diffractive grating.
Conventionally, an achromatic lens has been well known and used as a collimator lens or an objective lens. An example of the achromatic lens employs two thin lenses: one being a lens having large positive power that is made from crown glass having relatively low dispersion; and the other being a lens having small negative power that is made from flint glass having relatively high dispersion. The resultant power is therefore positive, while the dispersion is neutralized.
Since optical glass has dispersion such that refractive index thereof increases with decreasing wavelength of light, a positive refractive lens has longitudinal chromatic aberration where a back focus of the lens decreases as wavelength becomes shorter. The achromatic doublet corrects the chromatic aberration of the positive lens by using a negative lens that has opposite chromatic aberration.
The conventional achromatic doublet is designed for correcting the longitudinal chromatic aberration with respect to two different wavelengths, e.g., F-line (486 nm) and C-line (656 nm). That is, the light beams of F-line and C-line are focused on substantially the same focal point.
However, light beams having wavelengths except the C- and F-lines are not focused on the same focal point. Particularly, the back focus of the doublet becomes remarkably larger in a range shorter than F-line. This remaining chromatic aberration is called as secondary spectrum.
If the normal optical glasses are used for constituting the conventional type of the achromatic lens, it is impossible to correct the secondary spectrum sufficiently. Because refractive index of glass increases as wavelength becomes shorter as described, while increasing degree of the flint glass is larger than that of the crown glass in short wavelength range. Then the chromatic aberration of the doublet is overcorrected (backfocus becomes too long) in the range shorter than F-line.
In order to suppress the secondary spectrum of the longitudinal chromatic aberration with the conventional type of the achromatic lens, fluorite or anomalous dispersion glass should be used. However, these materials are too expensive.